It means that the addon has found two texture files matching suffixes for some texture file and do not know which one to use.
For example if you have MyMat_Color and MyMat_C in searched directory and for color texture type you have _Color and _C suffixes set then this error will be thrown.
But if you have only _C suffix set then only MyMat_C file will be found and no error will occur.
There are two PBR material workflows and they are not compatible with each other. One uses metallic and roughness textures while the other uses specular and glossiness textures (other textures like color or normals are common for both workflows).
This error appears if the addon detects textures of two different workflows in the directory and do not know which one to use.
You should have either only metallic and roughness textures or only specular and glossiness textures in the directory you have chosen.
If you want to know how exactly these two workflows differ I recommend this guide: link
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